It has become desirable to employ xerographic non-impact printers for text and graphics. An electrostatic charge is developed on the photoreceptive surface of a moving drum or belt and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat-fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing the photoreceptor surface. A row, or two closely spaced rows, of minute LEDs are positioned near a lens so that their images are arrayed across the surface to be illuminated. The LEDs along the line are selectively activated to either emit light or not as the surface moves past, thereby exposing or not exposing the photoreceptive surface in a pattern corresponding to the LEDs activated.
To obtain good resolution and image quality in such a printer, the physical dimensions of the LEDs must be quite small and very tight position tolerances must be maintained. One part of this concern is the spacing of the row of LEDs from the photoreceptive surface and the location of the lens therebetween to get a sharp image of the LEDs on the surface.
It has previously been the practice to simply assemble the row of LEDs with precision and build mounting structures that place the LEDs, lens, and photoreceptive surface at the best position obtainable by ordinary precision manufacturing techniques. There may, however, be variations in the mechanical dimensions of parts and differences in the optical properties of the lenses which result in the image on the photoreceptive surface being less than optimum.
It is, therefore, desirable to provide a technique for adjusting the optical elements of the print head for obtaining optimum image quality. The technique should be one that can be at least partly automated for use on an assembly line without requiring extensive judgment on the part of the operator and which does not require the making of test prints with an assembled printer. This latter is significant since the adjustments are preferably made on the print head independently of the balance of the printer.